I'm not sure why you're asserting that when you're completely wrong. Not only did Firefox support the old flexbox, it still does as of Firefox 22: [0] still works and uses the old (and prefixed) flexbox syntax, and said old (and prefixed) syntax remains documented on MDN with a note that it's deprecated[1]
[0] http://www.the-haystack.com/2010/01/23/css3-flexbox-part-1/
[1] https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-box-fl...
The old flexbox draft is similar to XUL boxes in some ways, and quite different in other ways. Trying to use XUL boxes and expecting the old-flexbox behavior as a result will land you in trouble, as happens with depressing regularity when people use "display: -moz-box".
The MDN documentation is sadly wrong, because it ignores the fact that the properties _look_ similar but have totally different behavior. Thank you for pointing that out; I'll make sure it gets fixed.